ISTANBUL — The United Nations on Monday revoked an invitation to Iran to attend a crucial peace conference on Syria in Switzerland, culminating a day of chaotic international diplomacy during which the United States objected to Iran’s inclusion and the Syrian opposition threatened to boycott.

Under intense American pressure, U.N. Secretary-General Ban Ki-moon rescinded the invitation to Iran, issued only 24 hours earlier, late on Monday, citing Iran’s failure to endorse the terms under which the conference was being held.

The last-minute decision appeared to take the U.S. and its European allies by surprise. An Iranian statement said Iran had accepted the invitation “without accepting any pre-conditions.”

According to Ban’s spokesman, Martin Nevirsky, the secretary-general had received reassurances from Iranian officials that they did accept the terms, spelled out in what is known as the Geneva 1 communique agreed on by Russia and the United States in 2012. But Iranian statements on Monday indicated that Iran did not accept the terms, which specify that the purpose of the conference is to negotiate a transition of power in Syria.

“In a series of meetings and telephone conversations, senior Iranian officials assured the secretary-general that Iran understood and supported the basis and goal of the Conference, including the Geneva Communique,” Nevirsky said in a statement read to reporters in New York. “The secretary-general is deeply disappointed by Iranian public statements today that are not at all consistent with that stated commitment.”

The announcement followed a day of intense diplomacy during which U.S. Secretary of State John Kerry personally lobbied Ban to rescind the invite, and the Syrian opposition warned that it would not attend if Iran was there, putting in jeopardy an event that has taken eight months of negotiations to bring to fruition.

The much-delayed peace conference will now go ahead on Wednesday with foreign ministers of more than 30 nations and continue later in the week with talks between negotiators for Syrian President Bashar Assad and representatives of the Syrian opposition.

Iran is Assad’s strongest regional ally and has supplied his government with advisers, money and materiel since the Syrian uprising began in 2011. The Islamic Republic’s allies, most notably the Lebanese Shiite militant group Hezbollah, have also gone to Syria to help bolster Assad’s forces.

Iran is prolonging the war and undermining chances for peace, and should not be allowed to sit at the peace table with diplomats from countries that are legitimately trying to end the war, a senior U.S. official close to the negotiations told The Washington Post earlier Monday.

U.N. officials have long argued that Iran is a critical player in the Syrian conflict and that any serious attempt to end the war must include officials from Tehran.

The disputes exposed the depth of the divisions within the international community over the Syrian conflict, which has killed more than 100,000 people over the past three years. Assad’s chief backers, Russia and Iran, have never endorsed the view shared by the U.S.and its European and Arab allies that the conference should be intended to negotiate an end to Assad’s rule.

In an interview with Agence France-Presse, Assad made it clear that he does not intend to relinquish power and said he planned to stand for re-election this year.

The University of Colorado leadership is grappling with how to address a nationwide nosedive in the favorability of higher education — particularly, among conservatives — as CU’s own representatives and decision-makers disagree on what’s behind the downturn.